STRUCTURAL CHALLENGES OF POWER PLANTS IN HIGH SEISMIC AREAS

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1 STRUCTURAL CHALLENGES OF POWER PLANTS IN HIGH SEISMIC AREAS Peter NAWROTZKI 1 and Daniel SIEPE 2 ABSTRACT Power plant buildings and machines can be protected against seismic demands utilizing proven 3- dimensional Base Control Systems (BCS) consisting of helical steel springs and viscous dampers. A BCS allows the tuning of rigid body mode shapes into the low frequency range in combination with an increase of structural damping. This results in significantly reduced seismic acceleration levels of the supported structures. For turbine buildings, the integration of the turbine substructure into the adjacent machine house structure can further reduce stress and strain values in all structural members. This contribution illustrates basic principles of elastic support systems and applications on power plant structures in high seismic areas. Some examples are offered showing the significant benefits and improved seismic performance of these structures. INTRODUCTION Safety in nuclear and non-nuclear facilities is of particular importance and recent seismic events and the corresponding damage in these facilities bring up the discussion on the structural challenges of these structures. Power plant equipment and power plant buildings have to be designed carefully for the load-case seismic input. The requirements related to earthquake stresses and strain are increasing worldwide. As a possible counter measure suitable seismic protection systems can be used to protect the structures and provide adequate safety margins. The present contribution puts emphasis on the reduction of structural responses due to seismic demands. The first part after the introduction covers general seismic protection strategies. Then, aspects of the elastic support of structures in seismically active regions and some details of already executed projects in the field of power plants are presented. Helical steel spring elements and viscous dampers can be used as a three-dimensional Base Control System (BCS) for structures. The spring devices provide elasticity and the dampers absorb the kinetic energy and are used to control the displacement of the structure and of the devices. The system frequency of the supported structure can be decreased and simultaneously, the damping ratio can be increased. Thus, seismically induced strain and stresses become smaller compared with reactions of unprotected structures. Possible damage after a severe earthquake can be reduced significantly, and the behaviour of the structural members could remain in the elastic range. The efficiency of a BCS can be shown by theoretical and experimental investigations as well as by comparing measured data of two identical real building structures with different support conditions (Stuardi et al., 2008). Additionally to the support of a whole building it is also possible to support individual equipment or structures inside a building by a Base Control System. Turbine generator sets represent one of the most important machinery in power plants. Placing these foundation decks on an elastic 1 Dr.-Ing., GERB Schwingungsisolierungen GmbH & Co.KG, Berlin, Germany, peter.nawrotzki@gerb.de 2 Dipl.-Ing., GERB Engineering GmbH, Essen, Germany, daniel.siepe@gerb.de 1

2 support system allows the integration of the turbine substructure into the machine building. Helical steel spring with special properties and highly efficient viscous dampers can be used here to reduce the relative displacement between the structures as well as the stress levels in all structural members due to seismic excitation (Nawrotzki, 2009). Concerning the type of individual spring and damper devices a wide variety is available. During the layout stage the most important structural frequencies of the entire system, the travel in the devices, the structural damping as well as important reactions (e.g. acceleration at the centre of gravity of the machine) are optimized. Thus, optimized parameters of the Base Control System could lead to further reduction of structural responses due to earthquake loading (Nawrotzki and Siepe, 2013). In some special cases it is also possible to update existing elements in case of increased seismic demands due to revised standards or regulations. The elastic support of power plant machinery is a well-accepted strategy for the dynamic uncoupling from their substructures. The flexibility of the spring devices has to be chosen in order to provide sufficient vibration isolation efficiency. The system is flexible in the horizontal directions, but also provides vertical elasticity. Thus, the devices, equipped with viscous dampers are used to protect the important machinery against seismic events at the same time. Details of some practical examples present the high efficiency of the suggested control system for the protection of equipment and buildings against seismic impacts in horizontal and vertical directions. SEISMIC PROTECTION STRATEGIES Placing a structure like a machine with its foundation block on top of a flexible support system leads to a modification of the fundamental mode shape. For the design of a vibration control system, considering the machine and the foundation as one rigid mass is usually sufficient, even if the machine itself is elastic. This is valid if the supporting devices are much more flexible than the machine and its foundation. The rigid body system will possess six low natural frequencies and corresponding mode shapes. In case of seismic excitation the change of the mode shape leads to smaller internal deflections and stresses of the structure itself compared to a structure with a rigid base. The modification of the mode shape as a first seismic protection strategy is usually combined with the reduction of the predominant frequency of the system. This second mitigation measure could be completed with the increase of structural damping. When the response spectrum curves of recorded or artificial earthquakes are plotted this strategy becomes obvious. The effects possible with passive control measures are shown in Fig.1. Here, the spectrum curves for a peak ground acceleration of 0.4 g, the soil class A, spectrum type 1 and three different damping values are plotted according to Eurocode 8 (DIN Deutsches Institut für Normung e.v., 2010). 1.2 HORIZONTAL ELASTIC SPECTRUM SPECTRAL ACCELERATION [g] FREQUENCY [Hz] D = 5.0 % D = 10.0 % D = 15.0 % Figure 1. Effects of frequency reduction and increase of structural damping The structural frequency is reduced from 4.0 Hz down to 1.0 Hz in this example. The viscous damping is increased simultaneously from 5.0 % to 15.0 %. The induced demands can be reduced to about 0.3 g in comparison to 1.0 g for the original natural frequency. The corresponding reduction of 2

3 P.Nawrotzki and D.Siepe 3 the structural responses causes a reduction of structural stress, strain, displacement etc. in a range of about 70.0 %. An optimum adjustment of frequencies and damping ratio by the use of a passive control system is required in order to reach such a significant improvement of the seismic behaviour of the protected structure. For each project, the boundary conditions should not be neglected. A very low frequency may yield too large displacements of the supported structure in the seismic case. An optimum between reduction of accelerations and corresponding displacements should be found. Support devices consisting of helical steel springs and viscous dampers are one type of passive control devices that are used for the previously described mitigation measures. More details of these systems will be given in the following chapter. ELASTIC SUPPORT SYSTEMS IN SEISMIC AREAS Spring elements with helical steel springs are used worldwide as an elastic support system. They possess linear-elastic behaviour in both horizontal and vertical directions. The elements are carrying the dead load of the structure and are designed to have sufficient safety margins to bear also additional loads from earthquakes. Viscodampers, installed beside the spring elements, have to absorb the kinetic energy. In general, they are used to dampen the relative motion between two structures, or between two parts of the same structure or between the spring supported structure and the rigid vicinity. The viscous damper provides linearly velocity dependant forces in all three spatial directions. The elastic support of a structure on springs and dampers leads to a three dimensional seismic protection system. This system is entitled as Base Control System (BCS) in order to distinguish it from well-described base-isolation systems, where horizontally very flexible and vertically very stiff elements are used. The combination of reduced frequencies and increased damping yields an efficient seismic protection in horizontal as well as in vertical directions. Accelerations are significantly reduced, resulting in less internal stresses of the supported structure. These positive effects are verified e.g. by Rakicevic et al. (2006) with theoretical and experimental investigations. Fig. 2 shows the arrangement of a BCS below a 3-storey building located in a high seismic zone of Argentina. The structure has the dimensions about 9 x 9 m in plan and consists of a reinforced concrete frame structure with infill brick walls. The entire structure is resting on a concrete slab that is supported by the spring and damper devices. The adjacent building was identically designed and built, but has got a rigid foundation. Both buildings were equipped with accelerometers. The recorded acceleration values during an earthquake in 2006 with a PGA of about 0.12 g verified the theoretical investigations that the BCS is very effective in reducing the seismic responses of the structure. The horizontal accelerations along the building height were significantly reduced. At the roof level they are reduced by more than 70 % (Stuardi et al., 2008). Figure 2. Seismic control system below apartment building

4 The design of a Base Control System depends on the target performance of the supported structure. For typical power plant machinery like a fan, a boiler feed pump or a diesel generator set the transmission of operational vibration should be mitigated first. Thus, the vertical flexibility of the steel springs has to be chosen in order to provide sufficient vibration isolation efficiency. Regarding the seismic demands of the project it is possible to adjust the parameters of the BCS. It should be added that the elements vary in the bearing capacity, in the horizontal and vertical stiffness properties, in the ratio between horizontal and vertical stiffness and in the damping resistance. EARTHQUAKE PROTECTION OF MACHINERY IN POWER PLANTS Earthquake control strategies of machinery are usually based on the change of the support conditions and corresponding reduction of structural frequencies in combination with increased damping. In the range of seismically significant frequencies many types of machines can be regarded as somehow rigid. Thus, a horizontally flexible layout with helical steel springs and additional damping devices can significantly improve the seismic performance of the supported machine. An important advantage of the spring devices is the fact that they can be adjusted and exchanged if required. The project that is described in the following section took advantage of this benefit. After the Fukushima event in 2011 the safety systems of the nuclear power plants had to be examined. One example is the nuclear power plant Gösgen, located in the north of Switzerland close to the village Däniken. This plant was built by the German supplier Kraftwerks Union (KWU) and is in operation since Diesel engines are one example for important machinery in a power plant. In case of failure of external power they have the task to supply power for all safety related systems. To increase the safety margin for the seismic case the existing spring and damper elements below the emergency diesels were replaced by new combined spring-damper elements. The total weight of the machinery, shown in Fig. 3, amounts to 38.0 tons and is supported by twelve spring devices. Earthquake protection is reached by low horizontal frequencies in a range of 1.0 Hz in combination with an increased damping ratio of more than 30.0 % for the corresponding modes shapes. Furthermore, the vertical flexibility of the devices has to be checked in regard to the vibration isolation efficiency of the machinery. To avoid any damage at the coupling or at the turbo charger connections of the diesel engine the seismic displacements have to be checked. Finally, an optimum between low frequencies and occurring displacements should be found. Figure 3. Emergency diesel generator set, NPP Gösgen, Switzerland 4

5 P.Nawrotzki and D.Siepe 5 The bunkered emergency diesel generators are part of the secured safety system. The existing spring and damper elements below these diesels were replaced by new combined spring-damper elements during the first shutdown of the plant after the Fukushima event (Kaulbarsch, 2013). The increase of the safety margin in a seismic case was the main reason for the replacement of the devices. The actual situation is shown on the left side in Fig. 4. The total weight of the structure amounts to approx tons, supported by four devices. The dimensions of the machine set are about 4 x 2 m in plan with a height of about 3 m. Each device consists of one viscous damper and several helical steel springs. The arrangement of the pre-stressable elements allows easy access during inspection and adjustment, if required. A similar seismic control system consisting of steel springs and viscous dampers is installed in a new building below the emergency power system which is required for the perimeter protection. Fig. 4 illustrates on the right side the arrangement of these devices. Figure 4. Vibration and seismic control devices for bunkered emergency diesel (left side) and emergency power system (right side), NPP Gösgen, Switzerland Turbo generator sets are an additional example for important machinery in a power plant. Fig. 5 shows on the left side a typical cross section layout of a turbine building without use of an elastic support of the turbine deck. The substructure of the machinery has to be completely separated from the adjacent machine house structure to avoid the transmission of vibrations during operation. This layout also leads to structural challenges if the power plant is located in a higher seismic zone. The sensitive machinery is located at the highest elevation of the separated structure. Thus, huge amplification factors between 2.5 and more than 3.0 are expected. The relative displacements between the separated structures will become significantly high and the connections like pipework systems should be designed for these motions. The columns and base mat below the machinery have to be very massive because no seismic damage of the structure is allowed. Conventional response reduction factors cannot be used for the design. Furthermore, the subsoil pressure below the separated pedestal becomes very high as the structure is tall and slender. Figure 5. Typical machine building without elastic support (left side) and building with integrated turbine deck on spring-damper system (right side)

6 Two steps can be considered for the seismic optimization of the structure. As a first step the building and the turbine foundation can share a continuous base slab. Vibration isolation devices have to be installed in this scenario. As a second step the connection between the substructure and the adjacent machine house structure is proposed. This layout is shown in Fig. 5 on the right side. Now, the direct substructure below the machinery is no longer part of the lateral load bearing system. The seismic behaviour corresponds more or less to a rigid body motion on top of the spring-column system. The structure could be easily designed for response reduction factor of R = 1.0. For the design of the columns and the base mat usual R-factors may be used, as these members are not connected directly to the machinery anymore. The continuous base slab leads to a better distribution of loads resulting in less problems in regard to the soil bearing capacity under seismic loading. The spring and damper system below the turbine foundation can be designed to achieve low frequencies and corresponding high values of the damping ratio. This seismic protection strategy provides numerous advantages. The relative displacements between substructure and machine house are reduced, the induced acceleration at the shaft level are decreased, the structural stresses are lowered and the demands of the machine building are reduced significantly. The layout of an integrated substructure as shown in Fig. 5 is used more and more worldwide. One typical corresponding example of a power plant in a seismic zone is presented in Fig. 6. The concrete deck for the turbo generator set is placed on helical steel springs with viscous dampers. The substructure consisting of steel members is integrated into the structural steel system of the adjacent machine building. The arrangement of spring elements and dampers allows the optimization of the seismic performance. Choosing laterally flexible springs reduce the amplification between peak ground acceleration and acceleration at the turbine deck compared to a conventional substructure without elastic support. Several parameters in regard to the target seismic performance have to be investigated during the design or qualification stage of the project. The stiffness of the system directly below the turbine deck, the stiffness of other parts of the machine building, the vertical stiffness of the elastic support, the ratio between vertical and horizontal stiffness of the spring devices and the influence of damping are usually examined. Figure 6. Elastically supported deck for a 600 MW turbine, Jhabua, India For this system an additional benefit of an elastic support system can be seen in reduced demands of the entire machine building. The system frequency is reduced and the damping ratio is increased, as the turbine deck itself acts as a mass damper for the complete structure. Here, the spring supported mass represents about 50.0 % of the entire structural mass. The seismic responses are reduced by about % (Basu et al., 2011). 6

7 P.Nawrotzki and D.Siepe 7 SEISMIC PROTECTION OF POWER PLANT BUILDINGS Based on the experience from the seismic protection of machinery it is possible to transfer the control strategies to the protection of different structures. In power plants parts of buildings and entire buildings have to be protected against possible seismic events. For such sensitive structures the effects of both horizontal and vertical excitations must be taken into account. Thus, it is required to use a control system that works efficiently in all three directions. A typical base-isolation system will lead to an extremely low horizontal frequency in combination with a high vertical stiffness. These parameters may lead to horizontal-vertical coupling effects that could amplify the horizontal accelerations (Ryan et al., 2012). The support on a control system consisting of helical steel springs and viscous dampers is proposed as an alternative solution. To assess the reduction effect of this 3-dimensional Base Control System a feasibility study was performed. Two different finite element models of a nuclear power plant building are prepared. The initial model consists of fixed restraints at the bottom plate. The second model considers the identical structure, but supported by springs and dampers below the base mat. The supported weight amounts to approximately tons. The properties of the devices are chosen taking into account horizontal and vertical effects. The structure is idealised as a 3-D finite element model. The longitudinal axis is the x-axis. The transversal axis is the y-axis. The z-axis is the vertical axis. Fig. 7 shows the FE-model and a typical picture of a spring element and a damper below a concrete structure. NODE 1 NODE 3 NODE 2 Figure 7. Finite element model of power plant building The numerical analysis in the time domain has been done using a ground acceleration time record. The time history data and the corresponding spectrum are shown in Fig. 8. As an assumption the same input is used for the horizontal and vertical directions. The calculation considers a simultaneous excitation in all three directions. Acceleration [g] Time [s] Spec. Acceleration [g] Frequency [Hz] Figure 8. Applied earthquake (left side) and corresponding spectrum for a damping ratio of 5.0% (right side)

8 The dynamic behaviour of both systems is investigated for the seismic load case. Table.1 shows a comparison of the results evaluated at different locations of the FE-model. In the horizontal directions it is possible to reduce the accelerations by more than 65 % when applying the seismic control system. As the system also functions in the vertical direction, there is a significant reduction effect otherwise not feasible. From these results it can be concluded that even the base shear and the internal stress and strain values will be reduced in a same order of magnitude Furthermore, small values of internal relative displacements are expected, where only the relative motion between building and surroundings is of particular importance. The corresponding connections, e.g. steam pipe systems, have to be designed to withstand such relative displacements. Table 1. Results of time-history analysis Without BCS With BCS Reduction [%] Max. absolute acceleration of node 1 in x-direction [g] Max. absolute acceleration of node 2 in y-direction [g] Max. absolute acceleration of node 3 in z-direction [g] Usually floor response spectra are calculated for the layout and design of machines or equipment located inside the building. Fig. 9 shows the efficiency of the BCS by comparing the floor response spectra at a higher elevation of the structure. For the first case ( RIGID ) the building is supported by fixed restraints at the base mat and in the second case ( BCS ) the same structure is placed onto a Base Control System. The frequency widening of the calculated spectra considers the assumption that the peak accelerations are constant for a frequency range of ± 15 % of the corresponding frequency. Due to the authoritative eigenfrequencies of the spring supported system a small narrow peak around these frequencies is unavoidable. In a wide frequency range the floor response spectrum of the building with BCS is significantly lower than the floor response spectrum of the unprotected building. Spec. Acceleration [g] X-Direction (Horizontal) D = 5.0 % 9.0 RIGID 8.0 BCS Frequency [Hz] Spec. Acceleration [g] Z-Direction (Vertical) D = 5.0 % 9.0 RIGID 8.0 BCS Frequency [Hz] Figure 9. Efficiency of BCS in horizontal and vertical direction It can be summarised that the arrangement of a Base Control System leads to a significant reduction of the accelerations, base reactions and spectral values of the floor response spectra in a wide frequency range. CONCLUSIONS After a short introduction into the fundamentals of seismic control strategies, some examples for the earthquake protection of power plant machinery and buildings are discussed in the present paper. Details of the applied Base Control Systems, consisting of helical steel springs and viscous dampers, 8

9 P.Nawrotzki and D.Siepe 9 for emergency diesel sets and turbo generator sets are used to explain the optimisation of the elastic devices, used already for providing vibration isolation. The proposed elastic support causes low system frequencies as well as high damping values. Thus, the Base Control System yields efficient earthquake protection of a structure by reducing accelerations and hence internal stress and strain values. REFERENCES Basu A K, Nawrotzki P, Siepe D (2011) Spring supported turbo-generator with steel supporting columns integrated into the steel structure of the turbine building, Proceedings of the POWER-GEN India & Central Asia, Delhi, India, 5-7 May DIN Deutsches Institut für Normung e.v. (2010) DIN EN Eurocode 8: Design of structures for earthquake resistance part 1: general rules, seismic action and rules for buildings, German version EN : AC:2009, Beuth Verlag GmbH, Berlin, Germany Kaulbarsch R (2013) Consequences of post-fukushima examinations, Proceedings of the SMiRT-22, San Francisco, USA, August Nawrotzki P and Siepe D (2013) Strategies for the seismic protection of power plant equipment, Proceedings of the International Conference on Seismic Design of Industrial Facilities, Aachen, Germany, September, Nawrotzki P (2009) Earthquake protection strategies for power plant equipment, Proceedings of the ASME Power 2009, Albuquerque, USA, July Rakicevic Z, Jurukovski D, Nawrotzki P (2006) Analytical modelling of dynamic behaviour of a frame structure with and without Base Control System, Proceedings of the 4 th World Conference on Structural Control and Monitoring, San Diego, USA, July Ryan K L, Dao N D, Sato E, Sasaki T, Okazaki T (2012) NEES/E-defense base-isolation tests: interaction of horizontal and vertical response, Proceedings of the Fifteenth World Conference on Earthquake Engineering, Lisbon, Portugal, September Stuardi J E, Nawrotzki P, Suarez L E (2008) Comparative seismic performance of a Base Control System based on measured and calculated responses, Proceedings of the Fourteenth World Conference on Earthquake Engineering, Beijing, China, October